Unless you are already familiar with regular expressions,
the discussion of special characters above probably looks forbiddingly
complex. A few more examples should make things clearer.
In the examples that follow, a square
() is used to mark a space; it is not a special character.

Let's work through how you might use some special characters in a replacement.
Suppose that you have a long file
and that you want to substitute the word child with the
word children throughout that file.
You first save the edited buffer with :w, then
try the global replacement:

:%s/child/children/g

When you continue editing, you notice occurrences of words
such as childrenish.
You have unintentionally matched the word childish.
Returning to the last saved buffer with
:e!, you now try:

:%s/child/children/g

(Note that there is a space after child.)
But this command misses
the occurrences child., child,, child: and so on.
After some thought, you remember that brackets
allow you to specify one character from among a list, so
you realize a solution:

:%s/child[,.;:!?]/children[,.;:!?]/g

This searches for child followed by either a
space (indicated by ) or any one of
the punctuation characters ,.;:!?. You expect to
replace this with children followed by the corresponding
space or punctuation mark,
but you've ended up with
a bunch of punctuation marks after every occurrence of children.
You need to save the
space and punctuation marks inside a \( and \).
Then you can "replay"
them with a \1. Here's the next attempt:

:%s/child\([,.;:!?]\)/children\1/g

When the search matches a character inside the \( and
\),
the \1 on the right-hand side restores the same character.
The syntax may seem awfully complicated, but this
command sequence can save you a lot of work!
Any time you spend learning regular expression syntax will be repaid
a thousandfold!

The command is still not perfect, though. You've noticed
that occurrences of Fairchild have been changed,
so you need a way to match child when it isn't part of another
word.

As it turns out, vi (but not all other programs that use regular
expressions) has a special syntax for saying "only if the pattern
is a complete word." The character sequence \<
requires the pattern
to match at the beginning of a word, whereas \> requires the
pattern to match at the end of a word. Using both will restrict
the match to a whole word. So, in the task given above,
\<child\> will find all instances of the word child,
whether followed by punctuation or spaces.
Here's the substitution command you should use:

If you want to save the prefixes, but want to change the name box to
square, either of the following replacement commands will do the trick.
The first example illustrates how \( and \) can be used to save whatever
pattern was actually matched. The second example shows how you can search
for one pattern but change another:

:g/mg\([ira]\)box/s//mg\1square/g

The global replacement keeps track of whether an i,
r or a is saved. In that way,
box is changed to square only when box is part of the
routine's name.

:g/mg[ira]box/s/box/square/g

This has the same effect as the previous command, but it is a little less safe
since it could change other instances of box on the same
line, not just those within the routine names.

You can also move blocks of text delimited by patterns.
For example, assume you have a 150-page reference manual.
Each page is organized into three paragraphs with
the same three headings: SYNTAX, DESCRIPTION,
and PARAMETERS.
A sample of one reference page follows:

.Rh 0 "Get status of named file" "STAT"
.Rh "SYNTAX"
.nf
integer*4 stat, retval
integer*4 status(11)
character*123 filename
...
retval = stat (filename, status)
.fi
.Rh "DESCRIPTION"
Writes the fields of a system data structure into the
status array.
These fields contain (among other
things) information about the file's location, access
privileges, owner, and time of last modification.
.Rh "PARAMETERS"
.IP "\fBfilename\fR" 15n
A character string variable or constant containing
the UNIX pathname for the file whose status you want
to retrieve.
You can give the ...

Suppose that it is decided to move DESCRIPTION above the SYNTAX
paragraph.
With pattern matching, you can move blocks of text on all 150 pages
with one command!

:g /SYNTAX/.,/DESCRIPTION/-1 move /PARAMETERS/-1

This commands works as follows. First, ex
finds and marks each line that matches the first pattern
(i.e., that contains the word SYNTAX).
Second, for each marked line, it sets . (dot, the
current line) to that line, and executes the command.
Using the move command,
the command moves the block of lines from the current line (dot)
to the line before the one containing the word
DESCRIPTION (/DESCRIPTION/-1)
to just before the line containing PARAMETERS
(/PARAMETERS/-1).

Note that ex can place text only below the line specified.
To tell ex to place text above a line, you first
subtract one with -1, and then ex
places your text below the previous line.
In a case like this, one command saves literally hours of work.
(This is a real-life example -- we once used a pattern match like this
to rearrange a reference manual containing hundreds of pages.)

Block definition by patterns can be used equally well with other ex commands.
For example, if you wanted to delete all DESCRIPTION paragraphs in
the reference chapter, you could enter:

:g/DESCRIPTION/,/PARAMETERS/-1d

This very powerful kind of change is implicit in ex's line
addressing syntax, but it is not readily apparent even to experienced
users.
For this reason,
whenever you are faced with a complex, repetitive editing task, take
the time to analyze the problem
and find out if you can apply
pattern-matching tools to get the job done.

Since the best way to learn pattern matching is by example,
here is a list of pattern-matching examples, with explanations.
Study the syntax carefully, so that you understand the principles at
work.
You should then be able to adapt these examples to your own situation.

Put troff italicization codes around
the word RETURN:

:%s/RETURN/\\fI&\\fP/g

Notice that two backslashes (\\) are needed in the replacement,
because the backslash in the troff italicization code will be
interpreted as a special character.
(\fI
alone would be interpreted as
fI;
you must type
\\fI
to get
\fI.)

Modify a list of pathnames in a file:

:%s/\/home\/tim/\/home\/linda/g

A slash (used as a delimiter in the global replacement sequence) must be
escaped with a backslash when it is part of the pattern or
replacement; use \/
to get /.
An alternate way to achieve this same
effect is to use a different character as the pattern delimiter.
For example, you could make the above replacement using colons as
delimiters. Thus:

:%s:/home/tim:/home/linda:g

This is much more readable.

Put HTML italicization codes around
the word RETURN:

:%s:RETURN:<I>&</I>:g

Notice here the use of & to represent the
text that was actually matched,
and, as just described, the use of colons as delimiters
instead of slashes.

Change all periods to semicolons in lines 1 to 10:

:1,10s/\./;/g

A dot has special meaning in regular expression syntax and must
be escaped with a backslash (\.).

Change all occurrences of the word help
(or Help) to HELP:

:%s/[Hh]elp/HELP/g

or:

:%s/[Hh]elp/\U&/g

The \U changes the pattern that follows to all uppercase. The
pattern that follows is the repeated search pattern, which is
either help or Help.

Replace one or more spaces with a single space:

:%s/*//g

Make sure you understand how the asterisk works as a special
character.
An asterisk following any character (or following any regular
expression that matches a single character, such as .
or [a-z])
matches zero or more instances of that character.
Therefore, you must specify two spaces followed by an asterisk
to match one or more spaces (one space, plus zero or more spaces).

Replace one or more spaces following a colon with two spaces:

:%s/:*/:/g

Replace one or more spaces following a period or a
colon with two spaces:

:%s/\([:.]\)*/\1/g

Either of the two characters within brackets can be matched.
This character is saved into a hold buffer, using \( and
\), and restored on the right-hand side by the \1.
Note that within brackets a special character such as a dot
does not need to be escaped.

Standardize various uses of a word or heading:

:%s/^Note[:s]*/Notes:/g

The brackets enclose three characters: a space,
a colon, and the letter s.
Therefore, the pattern
Note[s:] will match
Note,
Notes or Note:.
An asterisk is added to the pattern so that it also matches
Note (with zero spaces after it) and Notes: (the
already correct spelling). Without the asterisk,
Note would be missed entirely and Notes: would be
incorrectly changed to
Notes::.

Delete all blank lines:

:g/^$/d

What you are actually matching here is the beginning of the line (^)
followed by the end of the line ($), with nothing in between.

Delete all blank lines, plus any lines that contain only whitespace:

:g/^[tab]*$/d

(In the line above, a tab is shown as tab.)
A line may appear to be blank, but may in fact contain spaces or tabs.
The previous example will not delete such a line.
This example, like the one above it, searches for the beginning and end
of the line. But instead of having nothing in between, the
pattern tries to find any number of spaces or tabs.
If no spaces or tabs are matched, the line is blank.
To delete lines that contain whitespace but that
aren't empty,
you would have to match lines with
at least one space or tab:

:g/^[tab][tab]*$/d

Delete all leading spaces on every line:

:%s/^*\(.*\)/\1/

Use ^*
to search for one or more spaces at the beginning of each line;
then use \(.*\)
to save the rest of the line into the first hold buffer.
Restore the line without leading spaces, using \1.

Delete all spaces at the end of every line:

:%s/\(.*\)*$/\1/

For each line, use \(.*\)
to save all the text on the line,
but only up until one or more spaces
at the end of the line.
Restore the saved text without the spaces.

The substitutions in this example and the previous one
will happen only once on any given line, so the g option
doesn't need to follow the replacement string.

Insert a >
at the start of every line in a file:

:%s/^/>/

What we're really doing here is "replacing" the start of the line with
>.
Of course, the start of the line (being a logical construct, not an
actual character) isn't really replaced!

This command is useful when replying to mail or USENET news postings.
Frequently, it is desirable to include part of the
original message in your reply. By convention,
the inclusion is distinguished from your reply
by setting off the included text with a right angle
bracket and a couple of spaces at the start of the line. This can be done
easily as shown above. (Typically, only part of the original message will
be included. Unneeded text can be deleted either before or after the above
replacement.) Advanced mail systems do this automatically.
However, if you're using vi to edit
your mail, you can do it with this command.

Add a period to the end of the next six lines:

:.,+5s/$/./

The line address indicates the current line plus five lines.
The $ indicates the end of line. As in the previous
example, the $ is a logical construct. You aren't
really replacing the end of the line.

Reverse the order of all hyphen-separated items in a list:

:%s/\(.*\)-\(.*\)/\2-\1/

Use
\(.*\)
to save text on the line into the first hold buffer, but
only until you find
-.
Then use
\(.*\)
to save the rest of the line into the
second hold buffer.
Restore the saved portions of the line,
reversing the order of the two hold buffers.
The effect of this command on several items is shown below.

more - display files

becomes:

display files - more

and:

lp - print files

becomes:

print files - lp

Change every word in a file to uppercase:

:%s/.*/\U&/

or:

:%s/./\U&/g

The \U flag at the start of the replacement string tells
vi to change the replacement to uppercase.
The & character replays the text matched
by the search pattern as the replacement.
These two commands are equivalent; however, the first form is
considerably faster, since it results in only one substitution per
line (.* matches the entire line, once per line),
whereas the second form results in repeated substitutions on each line
(. matches only a
single character, with the replacement repeated on account of the
trailing g).

The search pattern matches all lines (a line contains zero or more
characters).
Each line is moved, one by one, to the top of the file (that
is, moved after imaginary line 0). As each matched line is
placed at the top, it pushes the previously moved lines down,
one by one, until the last line is on top.
Since all lines have a beginning, the same result can be achieved
more succinctly:

:g/^/mo0

In a database, on all lines not marked Paid in full,
append the phrase Overdue:

:g!/Paidinfull/s/$/Overdue/

or the equivalent:

:v/Paidinfull/s/$/Overdue/

To affect all lines except those matching your pattern,
add a ! to the g command,
or simply use the v command.

For any line that doesn't begin with a number, move the line
to the end of the file:

:g!/^[0-9]/m$

or:

:g/^[^0-9]/m$

As the first character within brackets, a caret negates the
sense, so the two commands have the same effect. The first one
says, "Don't match lines that begin with a number," and the
second one says, "Match lines that don't begin with a number."

The search string matches a digit other than zero, followed by a
period, followed by another non-zero digit.
Notice that the period doesn't need to be escaped in the replacement
(though a \ would have no effect, either).
The command above won't find chapter numbers containing
two or more digits. To do so, modify the command like this:

:%s/^[1-9][0-9]*\.[1-9]/.Ah/

Now it will match chapters
10 to 99 (digits 1 to 9, followed by a digit),
100 to 999 (digits 1 to 9, followed by two digits),
etc.
The command still finds chapters
1 to 9 (digits 1 to 9, followed by no digit).

Remove numbering from section headings in a document.
You want to change the sample lines:

2.1 Introduction
10.3.8 New Functions

into the lines:

Introduction
New Functions

Here's the command to do this:

:%s/^[1-9][0-9]*\.[1-9][0-9.]*//

The search pattern resembles the one in the previous example, but now
the numbers vary in length. At a minimum, the headings contain
number, period, number, so you start
with the search pattern from the previous example:

[1-9][0-9]*\.[1-9]

But in this example, the heading may continue with any number of
digits or periods:

[0-9.]*

Change the word Fortran to the phrase
FORTRAN (acronym of FORmula TRANslation):

:%s/\(For\)\(tran\)/\U\1\2\E(acronymof\U\1\Emula\U\2\Eslation)/g

First, since we notice that
the words FORmula and TRANslation use portions of the
original word, we decide to save the search pattern in two
pieces: \(For\) and \(tran\).
The first time we restore it,
we use both pieces together, converting all characters to uppercase:
\U\1\2. Next, we undo the uppercase with \E;
otherwise the
remaining replacement text would all be uppercase. The
replacement continues with actual typed words,
then we restore the first hold buffer. This buffer still contains
For, so again we convert to uppercase first: \U\1.
Immediately after, we lowercase the rest of the word: \Emula.
Finally, we restore the second hold buffer. This contains
tran, so we precede the "replay" with uppercase, follow it
with lowercase, and type out the rest of the word: \U\2\Eslation).